1. Field of the Invention
The present invention relates to a method of isomerizing n-butenes to isobutene or in the reverse reaction isobutene to normal butenes and the production of isobutene/n-butene codimer in a concurrent reaction distillation column reactor.
The process of concurrent reaction and distillation of the reaction catalyst as the distillation component is known as catalytic distillation and is described along with suitable catalyst structures in several commonly assigned U.S. patents and applications, including U.S. Pat. Nos. 4,215,011; 4,232,177; 4,242,530; 4,250,052; 4,302,356; 4,307,254 and 4,336,407.
Among the attributes of this process are:
(1) The utilization of a single reactor for carrying out the reaction and the initial distillation of the reaction mixture. Thus, initial investment in new equipment is substantially lower than liquid phase reactors and separate distillation towers. However, this aspect of the catalytic distillation gives rise to a further capital investment advantage in that the system can be retrofitted into existing distillation towers at a substantial savings in cost and time. This is an important consideration, since most refineries have unused or expendable distillation towers adequately suited for retrofitting.
(2) The catalytic reaction is very likely exothermic such as the etherifications and in those reactions where it is, the heat of reaction is used for the distillation (and supplemented by added heat as necessary).
(3) In exothermic reactions, the heat of reaction has been a problem, since in conventional reactions the heat must be dispersed or the reaction becomes a runaway, produces undesired by-products or certain types or catalyst can be damaged.
In the catalytic distillation, excess heat merely causes more boil-up and hence by controlling the pressure the temperature in the reactor is controlled and the heat of reaction is utilized as desired. Thus, in commercial scale the present process has proven very easy to start up, control and shut down. In other words, the reaction system has the ease and simplicity of operation of a distillation.
(4) The simultaneous reaction and distillation occurring within the catalyst structure results in disruption of reaction equilibrium and forces the reaction to completion.
It has now been found that the contact of isobutene with an acidic cation exchange resin catalyst result in the production of some n-butenes. Similarly, the contact of n-butenes (n-butene-1 and/or n-butene-2) results in the production of some isobutene. Since these are both reversible reactions they may or may not have occurred when mixed C.sub.4 streams were contacted with the catalyst, that is the presence of the various isomers in the C.sub.4 stream may surpress the isomerization or if it occurs, it is masked by the presence of the isomers.
In any event, the present invention provides the means to obtain the isomerization and recovery of the different isomers. The isobutene is important for the production of methyl tertiary butyl ether (gasoline octane improver), butyl rubber and the preparation of diisobutene which is used in detergent manufacture. The n-butenes are required in pure form for homopolymerization and as feeds for the oxidative production of butadiene.